Vacuum tight electromagnetic radiation permeable window seal



Nov. 10, 1964 R. J. BONDLEY 3,

VACUUM TIGHT ELECTROMAGNETIC RADIATION PERMEABLE WINDOW SEAL Filed Aug. 22, 1962 Fig.2.

Fig, 4.

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United States Patent 3,156,881 VACUUM TIGHT ELECTROMAGNETIC RADIA- TEON PERMEABLE WINDQW SEAL Ralph J. Bondley, Scotia, N.Y., assignor to General Electric Company, a corporation of New York Filed Aug. 22, 1962, Ser. No. 218,777 11 Claims. (Ci. 333-98) This invention relates to a hermetic seal transparent to electromagnetic radiation or other types of radiation and particularly to such a seal securely joined to metal waveguide or gas tight structure.

Demands for increased power at microwave frequencies emphasize the necessity for using low dielectric loss eramic, such as fused quartz, as a window for passing radio frequency energy. The hermetically sealed ceramic window material generally separates an electromagnetic wave generating means from means utilizing the electromagnetic radiation. The window permits evacuation of the generating means while providing an exit therefrom for this output energy.

Unfortunately, it is difficult to join many low expansion ceramic materials such as fused quartz in a permanent manner to a metal waveguide structure or the like. The ceramic window and metal waveguide combination must be capable of cycling through a large temperature range, not only in formation of the seal between the metal and ceramic but also during successive periods of operation. Fused quartz as a suitable ceramic material has a very low thermal coefiicient of expansion, e.g. .5 l0 per C., much lower than any metal usable with the ceramic material over wide temperature excursions. Since the quartz has a low mechanical strength in addition to its low thermal coefficient, a metal sealed to its outer periphery will pull away from the Window as the temperature rises.

It is therefore an object of the present invention to provide an improved vacuum tight window transparent to electromagnetic radiation and which is secure in construction and not subject to failure during temperature cycling.

In accordance with an exemplary embodiment of the present invention, a vacuum tight seal for passing electro magnetic radiation includes a central ceramic member and a concentric outer member spaced from the ceramic member. The outer member has a thermal coefficient of expansion which is relatively low but which is higher than that of the ceramic member. Means concentrically intermediate the outer member and the ceramic member has a thermal coefficient of expansion comparatively higher than the other materials and acts to expand inwardly as the temperature increases to compensate for the outward expansion of the outer member. The intermediate means therefore acts to securely support the central ceramic member.

In accordance with one aspect of the present invention, the ceramic member is employed to close off a section of tubular waveguide wherein tubular Waveguide houses the ceramic member. The means intermediate the ceramic member and the outer member then bears against the waveguide in juxtaposition with the ceramic member, restraining the waveguide from expanding away from the ceramic member.

In accordance with a feature of the present invention, the means intermediate to the ceramic member and the outer member comprises a plurality of metal spokes which bear against the ceramic member, or which bear against the waveguide housing the ceramic member, acting to tightly support the ceramic member. In accordance with another aspect of the invention, when the expansion of the outer member is not too great, a solid 3,156,8hl Patented Nov. 10, 1964 2 web of relatively high expansion material may be employed to securely engage the inner structure.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. The invention, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like reference characters refer to like elements and in which:

FIG. 1 is an end view of a first embodiment in accordance with the present invention,

FIG. 2 is a cross-section of the FIG. 1 structure taken at 2-2,

FIG. 3 is an end view of a second embodiment in accordance with the present invention, and

FIG. 4 is a cross-section taken at 44 in FIG. 3. V

The present invention includes a central member or disc-shaped window of low dielectric loss ceramic material and a concentric outer member having a low thermal coeflicient expansion but a coeflicient of expansion which is ordinarily greater than that of the ceramic material. Support means are included intermediate the outer member and the ceramic member having a higher thermal coefficient expansion for securely supporting-the ceramic member.

In the embodiment of FIGS. land 2, the central member 1 is fused quartz while outer member 2 is a strong but low-coefficient-of-expansion metal ring, and may be conveniently formed of molybdenum or tungsten. Ac-

cording to the embodiment of FIG. 1, the central ceramic member 1 is securely housed inside a metallic tubular waveguide or frame 3, preferably formed of low yield point conductive metal such as ,-silver and through which electromagnetic energy is passed. The waveguide or frame is Securely bonded to the periphery of the central ceramic member 1, for example with an indium-alloy metal bond as set forth in my copending application, Serial No. 741,713, filed June 11, 1958, and assigned to the assignee of the present invention. Briefly and by way of example, the edge of the central ceramic member is first painted with a fugitive adhesive such as polyisobutylone after which the edge is dusted with titanum hydride powder. Finely divided solder, e.g. indium, is then ap plied over the hydride after which the work area is evacuated and the temperature raised to approximately 550 C. to dissociate the hydride. In a matter of from 3 to 5 minutes at this temperature, the metals will have reacted with the quartz. The silver waveguide or frame 3 is then alloyed with the primarily indium surface edge at a temperature of approximately 850 C. to form the seal therebetween.

inter-mediate the outer member 2 and the central member 1 is disposed metal support means 4 constructed of high expansion metal material, the 300 series of stainless steels being appropriate as an example material. Further according to the embodiment of FIG. 1 the means 4 comprise a plurality of spokes which extend from the outer member 2 to the waveguide or frame 3 where they bear upon the central structure. The spokes must exhibit appropriate physical strength. An advantageous structurally secure shape therefor is the tapered configuration illustrated in FIG. 2 whereby the spokes decrease in axial width as they proceed inwardly from an outer member 4, the member 4 preferably being thicker in axial dimension than the central ceramic member 1. Where the spokes join the waveguide or frame 3, in juxtaposition with central member 1, they desirably have the same axial dimension as the central member. The spokes have a width which is preferably a fraction of their axial dimension.

mately-24.5 atomic percent nickel, at a-temperature some-.

what in excess of 955 C. The spokes may be joined to asilver waveguide or frame 3 with silver-copper eutectic solder. Other convenient soldering materials may be employed, or joining may be accomplished by pressure welding.

The device operates as follows: The radius of the outer member varies with temperature and becomes greater as the temperature increases. Means or spokes 4, because of the higher expansion coefficient, extend inwardly at a greater rate than the increase of the outer member. The inward expansion of means 4 provides a restraining force to support central member 1 and restrain the waveguide or frame 3 from leaving ceramic member 1 during temperature excursions, the waveguide or frame 3 being made of a soft or low yield point metal so the pressure variation between spokes is smoothed out before reaching the central member. Thus the outer member 2 and the intermediate supporting means 4 together provide a compensating action so as to protect the window seal; the waveguide, for example, is restrained to firmly engage the window, preventing any gas leakage therearound, while transmission of electromagnetic energy is fully facilitated through the window.

The device dimensions may be determined conveniently as follows:

Let

tzternperature change, r =inner radius of the outer member r =radius of the window w'=length of means 4 k =thermal coefficient of expansion of the outer member k =therrnal coefiicient of expansion of window k =thermal coefiicient of expansion of means 4 Thus the size change of the window radius with temperature is equal to tr k Similarly the size change of inner radius r is equal to tr k To meet the condition that the waveguide remain clamped against the window, means 4 must expand inwardly an amount substantially equal to the differences in size change between the window and the outer member, i.e.

Also

w:r -r Substituting t(r k -r k :tk (r -r Solving kjk t k -T lt); a kw: "F T T 1n ches Then w=r -r =0.42 inch.

The size and expansion coefficient of the thin waveguide structure do not materially affect the calculations regarding the particular illustrated embodiment, as com- .41. pared with the means 4, and therefore these factors have been neglected. The waveguide may be considered as a part of the means 4.

When the coefficient of the ceramic is close to that of outer member 2, a continuous metal web as an intermediate support means 4 is very satisfactory, and' is simpler to manufacture. This embodiment of my invention is illustrated in FIGS. 3 and 4 wherein the reference numerals refer to elements whose function and physical characteristics are substantially the same as elements similarly numbered in FIGS. 1 and 2. Therefore the general description of the device will not be restated.

In accordance with a particular example of this embodiment, outer member 2 formed of tungsten is joined as by soldering to supporting member 4 which comprises a solid web of copper. The window radius is 1.5 inches and the expansion coefficient for the particular ceramic is 3 X 10* per degree C.

As can be seen from the cross-section of FIG. 4, means 4- is tapered from outer member 2 towards the waveguide or frame 3 Where it firmly grasps frame 3 in juxtaposition with window 1 and where the frame is sealed to the ceramic. Means 4 is also soldered to waveguide 3 at this point. Substituting the thermal coefficients of expansion of the materials in the foregoing expression 5 yields:

In ractice the construction in accordance with the foregoing example firmly maintains waveguide 3 in its sealed relation to window 1 through successive temperature excursions between 80 C. and +600 C.

While. fused quartz as a window material is given as a frequent example in the foregoing description, it is understood the invention in its broader aspects is not limited thereto, nor is it limited to the metal materials mentioned. Moreover, the term electromagnetic radiation is intended in its broad sense to include forms of radiation in addition to radio frequency wave energy, for example, ultra-violet radiation.

While I have shown and described several embodiments of my invention, it will be apparent to those skilled in the art that many other changes and modifications may be made without departing from my invention in its broader aspects; and I therefore intend the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A vacuum tight seal transparent to electromagnetic radiation comprising a waveguide enclosed central member of low dielectric loss ceramic material having a low coefficient of expansion, an outer member having an intermediate coefficient of expansion and metal means between said waveguide and said outer member having a thickness greater than said waveguide acting to securely engage said central member and having a higher thermal coefficient of expansion so as to'expand inwardly with an increase in temperature.

2. A vacuum tight seal transparent to electromagnetic radiation comprising a waveguide enclosed central member of low dielectric loss ceramic material having a low coefiicient of expansion, an outer metal ring surrounding and spaced from said waveguide enclosed central member and having an intermediate thermal coefficient of expansion and a plurality of spokes extending from said outer member to said central member and acting to; se-

=l.66 inches curely support said central member having a length greater than the thickness of said waveguide, said spokes having a comparatively higher thermal coefficient of expansion.

3. The vacuum tight seal according to claim 2 wherein said central member is fused quartz, said outer member is chosen from the group consisting ofmolybdenum and tungsten wherein spokes are high expansion stainless steel.

4. A vacuum tight seal transparent to electromagnetic radiation comprising a disc of low dielectric loss ceramic material having a low thermal coeflicient of expansion, a tubular metal waveguide securely housing said disc wherein said waveguide is concentric to said disc and said disc closes 01f said waveguide, an annular outer member concentrically spaced from said waveguide, said outer member having an intermediate thermal coeificient of expansion and a plurality of spokes having a length greater than the thickness of said waveguide extending radially between said outer member and said waveguide in radial juxtaposition with both said outer member and said disc and for securely engaging said waveguide, said spokes having a relatively higher thermal coefficient of expansion so they expand inwardly towards said waveguide during a temperature rise as said outer member expands outwardly, thereby to restrain expansion of said waveguide away from said disc of ceramic material.

5. The vacuum tight seal transparent to electromagnetic radiation according to claim 4 wherein said outer member is axially thicker than said disc and wherein said spokes taper inwardly from said annular outer member to grasp said wave guide in juxtaposition with said disc.

6. The vacuum tight seal transparent to electromagnetic radiation according to claim 4 wherein said disc is fused quartz, said waveguide is low yield point conductive metal material, said annular outer member is chosen from the group consisting of molybdenum and tungsten and wherein said spokes are formed of a relatively high expansion metal material.

7. A vacuum tight seal transparent to electromagnetic radiation comprising a waveguide enclosed central member of low dielectric loss ceramic material having a low coefiicient of expansion, an outer member surrounding and spaced from said central member and having an intermediate thermal coeflicient of expansion and a metal web having a radial dimension greater than said waveguide extending from said outer member to securely support said central member in said waveguide, said web being formed of a metal having a relatively high thermal coefiicient of expansion.

8. A vacuum tight seal transparent to electromagnetic radiation comprising a central disc of low dielectric loss ceramic material, a tubular waveguide concentrically and securely housing said disc so that said disc closes off said waveguide, an annular outer member concentric with said disc and said waveguide while being spaced from said waveguide, said outer member being formed of a material having an intermediate thermal coefiicient of expansion, and a metal web between said outer member and. said waveguide having a radial dimension greater than said waveguide and having a relatively higher expansion coefficient securely supporting said waveguide whereby said Web expands inwardly with an increase in temperature as said outer member expands outwardly.

9. The vacuum tight seal transparent to electromagnetic radiation according to claim 8 wherein said annular outer member is greater in axial thickness than said ceramic disc and wherein said Web tapers inwardly from said annular outer member to grasp said waveguide in juxtaposition with said ceramic disc.

10. The vacuum tight seal transparent to electromagnetic radiation according to claim 8 wherein said disc is fused quartz, said waveguide is a conductive low yield point metal, said annular outer member is a low expansion metal chosen from the group consisting of molybdenum and tungsten and wherein said Web is relatively higher expansion metal chosen from the group consisting of copper and stainless steel.

11. A vacuum tight seal transparent to electromagnetic radiation comprising: a central disc of low dielectric loss ceramic member having a radius r, and a coeflicient of expansion k an outer member concentrically surrounding and spaced from said disc, said frame being formed of a material having an intermediate thermal coefiicient of expansion, k and having an inner radius r,,; and metal means extending substantially between said disc and said outer member, having a relatively higher thermal coefficient of expansion k equalling References Cited in the file of this patent UNITED STATES PATENT-S 3,058,074 Kane Oct. 9, 1962 

1. A VACUUM TIGHT SEAL TRANSPARENT TO ELECTROMAGNETIC RADIATION COMPRISING A WAVEGUIDE ENCLOSED CENTRAL MEMBER OF LOW DIELECTRIC LOSS CERAMIC MATERIAL HAVING A LOW COEFFICIENT OF EXPANSION, AN OUTER MEMBER HAVING AN INTERMEDIATE COEFFICIENT OF EXPANSION AND METAL MEANS BETWEEN SAID WAVEGUIDE AND SAID OUTER MEMBER HAVING A THICKNESS GREATER THAN SAID WAVEGUIDE ACTING TO SECURELY ENGAGE SAID CENTRAL MEMBER AND HAVING A HIGHER THERMAL COEFFICIENT OF EXPANSION SO AS TO EXPAND INWARDLY WITH AN INCREASE IN TEMPERATURE. 